Integrated photonics has proved to be a useful technology for transmitting, receiving, routing and processing of information in optical form as is widely documented in an extensive literature. Typically, integrated photonic devices require the controlled coupling of light between waveguides formed in an integrated chip. It is known in the prior art that a useful method for performing this coupling is to position two waveguides in parallel fashion with a controlled separation allowing light to couple the waveguides 1 and 2 across the gap 3 formed therebetween as shown in FIG. 1. The properties of this type of coupling are determined by the material, shape and dimensions of the waveguides, cladding parameters, and the dimension of the coupling gap 3 which is particularly important in determining the coupling strength between the waveguides 1 and 2.
Typically, integrated photonics fabrication is based on the planar processing technology developed for semiconductor integrated circuits, while layers of material are deposited or grown upon largely planar substrate surfaces. Structures are patterned along the surface of the plane using a succession of lithographic processes combined with various etching, deposition, alloying, implantation and other well-known techniques.
Integrated optical waveguides may be fabricated in the following ways: a) with dimensions perpendicular to the plane (which is referred to herein as the vertical direction) generally determined by the thickness of grown or deposited layers with values being set by the growth or deposition process parameters, or (b) with the in-plane dimensions (which are referred to as lateral directions) generally determined by lithographic patterns as they define the regions where other additive, subtractive or modifying processes are applied.
It has been noted that coupling of two waveguides 4 and 5 in a manner that the waveguide separation, or coupling gap 6 is oriented in the vertical direction (referred to as vertical coupling) shown in FIGS. 2A and 2B has several advantages.
Initially, the waveguide separation in vertically coupled structures is determined by a layer growth or deposition process rather than by a lithographic process. Since it is frequently possible to control the thickness and material characteristics of a grown or deposited layer to a higher degree of accuracy and precision than that of a lithographic process, an enhanced control over the coupling strength between vertically separated waveguides is attained.
Secondly, vertical coupling geometry has an advantage of greatly enhanced alignment tolerance between the waveguides to be coupled. As described previously, the coupling strength between two waveguides is a highly dependent function of the waveguide separation and in many cases the relationship has an exponential dependence.
In conventional integrated couplers in which two waveguides 1 and 2 are separated in a lateral direction (as shown in FIG. 1), a relatively small variation in the waveguide separation 3 due to lithographic nonuniformity or misalignment may cause a considerable change in the coupling strength. In the case of vertical coupling shown in FIGS. 2A and 2B, the maximum coupling strength occurs when two waveguides 4 and 5 are optimally aligned in the lateral direction. At such an optimum point, the dependence of coupling on misalignment (lateral displacement 7) is stationary to a first order (the first derivative is zero at a maximum point). Thus, the coupling can be made relatively insensitive to small errors in the lateral alignment 7 of vertically coupled waveguides.
Microring resonators proved to be promising building blocks for very large-scale integrated optics. In the past few years, single microring resonators laterally coupled to a waveguide have been fabricated in Si—SiO2 and GaAs—AlGaAs. Advanced functions thereof have been demonstrated such as high-order filtering for DWDM applications, notch filters and wavelength conversion. These devices were based on lateral coupling between the waveguides and the ring fabricated by lithography. In these devices it is critical to make the separation between the waveguides and the microring smaller than 0.3 μm. Disadvantageously, lithography may fail to deliver such preciseness of parameters, thus making reproducible bandwidth and high power dropping efficiency of the integrated optical devices difficult to achieve.
This problem is substantially alleviated with vertical coupling by controlling the sensitive separation between optical components with material growth or deposition, and incorporating the inherently symmetric structure of the integral devices. Waveguides and ring resonators that take advantage of vertical coupling have been demonstrated which were fabricated in compound glass, silica and in semiconductors. The process included sequential deposition (regrowth) of several waveguiding layers, one on top of another, with patterning of each layer before deposition (regrowth) of a subsequent layer. This technique required multiple redeposition (or regrowth) and planarization which has suffered from unwanted complexity.
A regrowth-free fabrication technique was developed which allowed fabrication of vertically coupled structures in semiconductors using wafer bonding, substrate removal and infrared backside contact alignment. The regrowth-free technique, however has suffered from a number of drawbacks. First, the method used contact lithography techniques outfitted with infrared alignment systems which are able to do accomplish alignments and exposures, however, the method has serious limitations in resolution and inter-level alignment accuracy as compared to projection systems. Due to the limitations in resolution and alignment presented by infrared backside-aligned contact photolithography, it was difficult to attain highly confined vertically coupled structures.
Secondly, due to the fact that alignment marks on the original top surface of the structure would not be visible to a conventional projection stepper after the wafer bonding and substrate removal process, the direct write electron beam lithography or deep UV photolithography was necessitated to achieve a deep sub-micron alignment tolerance which was still a requirement between waveguides even in the case of vertically coupled devices. Thus, this technique was not compatible with projection lithography which is a known technique well suited for widespread use of large scale silicon integrated circuit fabrication.
It has become apparent that a simpler technique for fabrication of integrated photonic devices compatible with projection lithography for manufacturing of highly confined waveguides in semiconductors with the sub-micron alignment tolerance between waveguides is still necessary in the field of integrated photonics.